In U.S. Pat. No. 3,899,295 and U.S. Pat. No. 5,096,813 indicator systems are disclosed in which a sensor contained within a package changes colour if the package is tampered with and a gas enters into or escapes from the package. In U.S. Pat. No. 3,899,295 the system provides for the addition of a basic gas to a package before it is closed and a sensor contained within the package changes colour when the basic gas is released if the package is tampered with. In U.S. Pat. No. 5,096,813 the system disclosed includes a sensor contained within the package which changes colour if the package containing no oxygen is tampered with such that oxygen enters into the package.
Although the systems disclosed in U.S. Pat. No. 3,899,295 and U.S. Pat. No. 5,096,813 provide a gas indicator within a package which changes colour when a gas enters in or passes from a package, the systems do not provide for an indication of any change of atmosphere within the package if it is not tampered with.
Furthermore, the system disclosed in U.S. Pat. No. 3,899,295 would not function for MAP because MAP does not involve the addition of a basic gas. The main gas used in MAP is carbon-dioxide, which is acidic. It is the acidity of carbon dioxide used in MAP that provides packaged food with a long shelf life. The addition of a basic gas will offset and ruin this effect.
Similarly, the system disclosed in U.S. Pat. No. 5,096,813 would not be suitable for MAP in which the atmosphere contained in a package included oxygen. More importantly, however, the system is not suitable for MAP because it is not reversible and therefore does not provide the facility for indicating increase and decrease of the proportion of a gas in the atmosphere contained within a package. The system also would be difficult to process because of the short time available during which the sensor material could be exposed to air. Furthermore, the period stated in U.S. Pat. No. 5,096,813 of one to eight hours for colour change of a gas sensor to take effect is much too slow for MAP where it is required that such colour change should be effective within seconds or minutes.
Generally, in MAP an envelope containing a food product is flushed with a specific mixture of gases and the envelope is then sealed. Usually the gases are a combination of carbon dioxide, oxygen and nitrogen, but the atmosphere may consist of only a single gas. For example, red meat is often packaged under a mixture of 20% carbon dioxide and 80% oxygen; poultry is often packaged under a mixture of 25% carbon dioxide and 75% nitrogen; cheese is often packaged under 100% carbon dioxide; and fresh pasta is often packaged under 100% nitrogen.
A problem with MAP is that it is difficult to ensure that a correct gas mixture is provided in a package. Generally, screening is carried out by testing a small percentage of a batch of packages and, if any of the tested packages are found to be defective, the whole batch of packages is discarded including many untested and satisfactory packages included in the batch. More importantly, there is no indication that and atmosphere initially established is still present in a package. If the package is opened or tampered with and gases contained therein escape, generally the first indication that the package is defective is when the product shows signs of deterioration.